I found a good definition at Wikipedia, and although not mentioned here, it is frequently called the Read/Write Web, considering the "original" web, now labeled Web 1.0, being pretty much one way. The quick definition is here, but the link to the full article contains much more info:
From Wikipedia, the free encyclopedia
Web 2.0, a phrase coined by O'Reilly Media in 2004, refers to a supposed second-generation of Internet-based services—such as social networking sites, wikis, communication tools, and folksonomies—that emphasize online collaboration and sharing among users. O'Reilly Media, in collaboration with MediaLive International, used the phrase as a title for a series of conferences and since 2004 it has become a popular (though ill-defined and often criticized) buzzword amongst certain technical and marketing communities.